In personal communication service (PCS) systems, it is necessary to low frequency phase synchronize the clocks of the wireless base stations. The clocks of the base stations must be synchronized at a precise low frequency such as 0.488 Hz. In PCS systems, this is normally done by allowing one of the base stations to be the master and periodically transmitting the synchronization information via a wireless channel. The problem with this method is that all base stations must be able to receive the transmitted synchronization signals. In larger PCS systems, it is not possible for one base station to serve as the master with respect to synchronization since no one base station can broadcast to all other base stations. A second method that has been utilized in PCS systems, is to have a separate wired distribution system that provides the synchronization signals to each of the base stations. The problem with this technique is the added cost to the PCS system. A typical base station in a PCS system only handles a maximum of three wireless handsets at a time; hence, there are a large number of base stations to cover a large geographical area. This large number of base stations increases the cost of providing a separate wired distribution system for the synchronization signals.
In cellular systems in which a base station handles hundreds of wireless handsets and where the synchronization must occur at approximately 8 kHz, one technique employed to perform the synchronization is to utilize a earth-orbiting satellite to broadcast a synchronization signal. Another technique in cellular systems is to have wire or optical fiber links separate from the normal, customer traffic, communication links to connect the base stations to a central controller. The central controller transmits the synchronization signals to the base stations over the links. In yet another technique, central controller periodically stops the normal communication on the normal communication links leading between the base stations and the central controller, then broadcasts synchronization signals on the links, and then resumes normal communication. While avoiding the expense of separate dedication synchronization links, this technique causes periodic interruptions in normal communications.
Another technique for cellular base stations is disclosed in U.S. Pat. No. 5,388,102. The method disclosed in this patent requires that the ISDN interface cards utilized in the telecommunication switching system interconnected to the base stations be modified so that the synchronization signals can be transmitted in the physical layer protocol of the ISDN interface. In addition, that method also requires that the internal bus structure of the telecommunication system be modified to provide the necessary timing to the modified ISDN interface cards. The problem with this method is that it does not adapt well to use with a variety of telecommunication switching systems.
What the prior arts lacks is a simple and inexpensive technique for supplying base station synchronization signals in a PCS system.